The present embodiments relate to a method, a device and a computer program for adjusting the visualization of volume data of an object as an image.
The use of X-rays in medical diagnosis is a widely established practice. Thus, for example, the examination of female breast tissue for the formation of carcinomas may be carried out using X-ray radiation (e.g., mammography).
Owing to the specific anatomical features of the region of the body being examined, special-purpose devices, which may be referred to as mammography devices, are used for examinations of the type using X-rays.
Projection settings of mammography devices have become established as standard settings for diagnostic purposes. The following two standard settings may be used.
The mediolateral oblique (MLO) view of the breast (e.g., oblique projection) is the standard setting employed in the early detection of breast cancer by mammography. An image of the breast is recorded at a 45° angle. The 45° oblique projection is intended to image the upper outer quadrants, the axillary tails and the inframammary folds.
Alongside this, there is also the craniocaudal (CC) view, in which an image of the breast is recorded vertically from above. The CC projection may show as much breast tissue as possible and ideally images all breast sections except for the sections in the furthest lateral and axillary positions.
A procedure known as 2-plane mammography, which combines the mediolateral oblique (MLO) view and the craniocaudal (CC) view, may be carried out within the course of a standard examination.
Despite this combination of projections taken from different angles, conventional mammography has its limits. There is the risk that tissue hardening (e.g., calcifications) is concealed in the X-ray image by other structures and is not diagnosed.
Tomosynthesis, which is employed, for example, in digital mammography, provides improved diagnostic possibilities. In contrast to computed tomography, tomosynthesis is based on only one comparatively small angular interval being scanned in the course of the movement of the X-ray tube around the object that is to be examined. The restriction of the interval may be determined by the examination object (e.g., female breast).
A sequence of tomosynthesis projections in mammography may be acquired by a modified mammography system or a breast tomosynthesis system. In this case, for example, 25 projections are taken while the X-ray tube moves over the detector in an angular range between −25° and 25°. The radiation is triggered at regular intervals during this movement, and one projection is read out from the detector each time. A three-dimensional representation of the examined object is subsequently reconstructed in the computer from the projections in a tomosynthesis reconstruction process. The object may be present in the form of grayscale values that constitute a metric for the density at voxels or points in space associated with the grayscale values. The Z layers of the reconstructed volume (e.g., reconstructed slice images that are aligned parallel to the detector plane) are examined in most cases in the course of the medical diagnosis.
An improvement in the examination of Z layers may be achieved using visualization techniques for three-dimensional volume datasets.
Techniques collectively known as volume rendering are employed in order to represent three-dimensional volumes as an image on a monitor. One example of such a technique (e.g., direct volume rendering) is ray casting (e.g., the simulation of rays penetrating the volume). Another technique is, for example, multiplanar reformation (e.g., multiplanar reconstruction (MPR)). This is a two-dimensional image reconstruction method, in which raw data present as transversal slices is used to compute frontal, sagittal, oblique or curved slices that assist the viewer in the anatomical orientation. In the maximum intensity protection (MIP) method, the point having the maximum grayscale value from the 3D volume along the observational axis is imaged directly in each case. A two-dimensional projection image is generated. A spatial context is created in this way when a series of MIP images is viewed from different observer positions. This method may be used for visualizing structures filled with contrast agent.
The application of methods of this type for visualizing tomosynthesis data is described, for example, in the publications US 20100166267 A1, US 20090034684 A1, U.S. Pat. No. 7,760,924 and US 20090080752 A1.
With all these methods, it is taken into account that a large bandwidth of different density (and hence a further range of grayscale values) occurs in the volume data that may be present in the form of grayscale values. A scale named for the scientist Hounsfield and extending approximately from −1000 (e.g., for lung tissue) to 3000 (e.g., for bone) may be used to describe the reconstructed attenuation values. A grayscale level is assigned to every value on this scale, resulting in a total of approximately 4000 grayscale levels to be visualized. This scheme, which is customary in CT for three-dimensional image constructions, may not simply be transferred to monitors used for visualization purposes. One reason for this is that no more than 256 (e.g., 28) grayscale levels may be visualized on a commercially available 8-bit monitor. There is little point in representing a higher number of grayscale levels because the imaging granularity of the display already significantly exceeds that of the human eye, which may distinguish approximately 35 grayscale levels. Efforts are therefore directed at extracting the diagnostically relevant details for the purpose of visualizing human tissue. One possibility for this is the definition of windows encompassing a particular grayscale value range at a level that is relevant for the diagnosis. A term also employed in this context is “window leveling.” Histologically calcified lesions, for example, may have grayscale values in the range of approximately 500 Hounsfield units. In order to diagnose such calcifications (e.g., in mammography), a window may be set in a range around 500 Hounsfield units. With this approach or, more specifically, this window, the adipose and connective tissue of the breast, which lies in the negative Hounsfield unit range, may disappear. A similar situation arises in the case of volume rendering, in which the relevant structures are made visible by transfer functions that map grayscale values to color values and permeability coefficients (e.g., opacity values). With that technique, the adipose and connective tissue may be rendered as transparent so that calcifications may be seen.
With this approach, it is taken into account that the diagnosis of malignant changes is a complex undertaking. Thus, larger calcifications may be benign, while smaller calcifications (e.g., microcalcifications) are indicators of a tumor formation. In order to arrive at a better assessment, the physician requires as much relevant information as possible about the region of the tissue transformation and the embedding of the changed tissue in the surrounding tissue layers.